Thermoelectric generator and method of generating electricity

ABSTRACT

A generator device for converting thermal energy to electric energy. A magnetic circuit includes at least a portion made of a magnetic material. A temperature-varying device varied the temperature in the portion made of the magnetic material alternately above and below a phase transition temperature of the magnetic material to thereby vary the reluctance of the magnetic circuit. A coil is arranged around the magnetic circuit, in which electric energy is induced in response to a varying magnetic flux in the magnetic circuit. A magnetic flux generator creates magnetic flux in the magnetic circuit. The temperature-varying device alternately passes hot and cold fluid by, or through holes in, the portion made of the magnetic material of the magnetic circuit in a single direction to thereby vary the temperature in the portion made of the magnetic material alternately above and below the phase transition temperature of the magnetic material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Swedish patent application 0700780-0filed 28 Mar. 2007 and is the national phase under 35 U.S.C. §371 ofPCT/EP2008/053215 filed 18 Mar. 2008.

TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to a generator device and methodfor converting thermal energy to electric energy.

DESCRIPTION OF RELATED ART AND BACKGROUND OF THE INVENTION

Apparatuses for transforming thermal energy directly into electricenergy using a minimum of moving parts have been known since long.

At the end of the 19'th century, Edison and Tesla described devicesbased on thermomagnetic materials for converting thermal energy intoelectricity. Edison's pyro-magnetic generator as described in U.S. Pat.No. 380,100 includes a thermomagnetic working material, means formagnetizing the working material, sources of heat and cold connected tothe working material, and a winding enclosing the working material andin which an alternating electric current is induced by thermally cyclingthe working material. Tesla discloses in U.S. Pat. No. 428,057 someimprovements of Edison's generator by suggesting an alternative heatexchanging mechanism.

Chilowsky discloses in U.S. Pat. No. 2,510,801 a device forthermomagnetic energy conversion, wherein the temperature variations areachieved by hot and cold fluids in a closed fluid circuit.

Bartels discloses in German patent No. 23 47 377 a device similar toEdison's generator, but suggests gadolinium as magnetic working medium.In one embodiment a permanent magnet is provided to create the magneticflux in the thermomagnetic material and in another embodiment a batteryis used to induce a current in a coil to thereby form an electromagnetto create the magnetic flux.

While the apparatuses described above do convert thermal energy directlyto electric energy, they seem not to be efficient, compact, optimizedfor practical use, and/or cost-efficient to manufacture.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a generator devicefor converting thermal energy to electric energy, which has asimplified, yet improved, structure and operation.

It is a particular object of the invention to provide such a device anda method for converting thermal energy to electric energy, which use asmooth and energetically efficient thermal cycling.

It is still a further object to provide such a device and method, whichare dynamically controllable, reliable, flexible, and of reasonablecost.

These objects, among others, are according to the present inventionattained by generator devices and methods.

According to one aspect of the invention there is provided a generatordevice for converting thermal energy to electric energy comprising amagnetic circuit including at least a portion made of a magneticmaterial, and a coil arranged around the magnetic circuit. Atemperature-varying apparatus is provided for varying the temperature inthe portion made of the magnetic material alternately above and below aphase transition temperature of the magnetic material to thereby varythe reluctance of the magnetic circuit. A magnetization of each of themagnetic circuits is modulated by the varying reluctance so as to induceelectric energy in the coil arranged around the magnetic circuit.Examples of such phase transition temperatures are the Curie point ortemperature and the Néel temperature. The Curie point of a ferromagneticmaterial is the temperature above which it loses its characteristicferromagnetic ability. The Néel temperature is the temperature at whichan antiferromagnetic material becomes paramagnetic—that is, thetemperature at which the thermal energy becomes large enough to destroythe macroscopic magnetic ordering within the material. The Néeltemperature is analogous to the Curie temperature for ferromagneticmaterials.

The temperature-varying apparatus is provided to alternately pass hotand cold fluid by, or through holes in, the portion made of the magneticmaterial of the magnetic circuit in a single direction to thereby varythe temperature in the portion made of the magnetic material alternatelyabove and below the phase transition temperature. Preferably, thetemperature-varying apparatus is provided to circulate the hot and coldfluid as a quasi-continuous flow in a closed fluid loop. Thereby,traditional disruptive and energetically inefficient cycling usingvalves is avoided.

In one embodiment of the invention the temperature-varying apparatuscomprises first and second valve arrangements. The first valvearrangement is adapted to alternately switch hot fluid from a commonfirst pipe and cold fluid from a common second pipe into a third pipeand expose the portion made of the magnetic material of the magneticcircuit for the alternately hot and cold fluid. In other words, thefirst valve arrangement divides up the hot fluid and the cold fluid in afluid pulse train of alternating hot and cold fluid pulses. Such a fluidtrain is directed to the magnetic circuit in order to heat and cool itsportion made of the magnetic material.

The second valve arrangement is adapted to receive the fluid pulse trainafter having alternately heated and cooled the portion made of themagnetic material and to switch the hot fluid pulses into the commonfirst pipe and the cold fluid pulses into the common second pipe. Inother words, the second valve arrangement divides up the hot and coldfluid pulses and forms from them a hot fluid flow and a cold fluid flow.

Advantageously, the first and second valve arrangements include each arotating valve.

The temperature-varying apparatus is advantageously used in a multiphasegenerator device comprising a plurality of the magnetic circuitdescribed above and a respective coil arranged around each of themagnetic circuits.

According to another aspect of the invention there is provided a methodfor converting thermal energy to electric energy in a generator devicecomprising a magnetic circuit including at least a portion made of amagnetic material and a coil arranged around the magnetic circuit. Inthe method the temperature is varied in the portion made of the magneticmaterial alternately above and below a phase transition temperature ofthe magnetic material to thereby vary the reluctance of the magneticcircuit. Each of the magnetic circuits is magnetized and due to thevarying reluctance a correspondingly varying magnetic flux is obtainedin the magnetic circuit. As a result thereof, electric energy is inducedin each of the coils. The temperature variations are obtained by meansof alternately passing hot and cold fluid by, or through holes in, theportion made of the magnetic material of the magnetic circuit in asingle direction.

The present invention is suitable for a large variety of electric powergeneration by using any of waste heat, combustion heat, thermal storagereservoir energy, geothermal energy, solar radiation, solar thermalenergy, ocean thermal energy, or energy from nuclear reactions.

According to yet another aspect of the invention there is provided arotating valve having a cylindrical casing, a shaft arrangedsymmetrically in the casing, a member, e.g. blades or a disc, fixedlyattached to the shaft and in close fit with the cylindrical casing,thereby defining essentially separated chambers within the casing, aplurality of outlets or inlets fixedly arranged along the circumferenceof the casing, and a plurality of axially arranged inlets or outlets,each of which being fixedly connected to a respective one of theseparated chambers. The separated chambers and thereby the axiallyarranged inlets or outlets are alternately in fluid connection with eachof the outlets or inlets fixedly arranged along the circumference of thecasing in response to rotation of the shaft and the member with respectto the casing.

Such a rotating valve can be used for the thermal cycling of fluid in athermomagnetic generator device, but can alternatively be used inentirely different applications, in which fluids of differentcharacteristics should be alternately output in a single pipe.

Further characteristics of the invention and advantages thereof, will beevident from the following detailed description of preferred embodimentsof the present invention given hereinafter and the accompanying FIGS.1-4, which are given by way of illustration only and thus, are notlimitative of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 displays schematically a thermomagnetic generator deviceaccording to an embodiment of the present invention.

FIG. 2 displays schematically a thermomagnetic generator deviceaccording to a further embodiment of the present invention.

FIG. 3 displays schematically in a top view an example of a rotatingvalve as usable in the thermomagnetic generator device of FIG. 2.

FIG. 4 displays schematically in a perspective view another example of arotating valve as usable in the thermomagnetic generator device of FIG.2.

DETAILED DESCRIPTION OF EMBODIMENTS

A thermomagnetic or magnetothermal generator device for directtransformation of heat into electric energy according to an embodimentof the present invention comprises, as shown in FIG. 1, a magnetic ringor circuit 1, a temperature-varying device 5, and a coil or winding 7arranged around the magnetic circuit 1.

The magnetic circuit may be substantially of iron or other magneticmaterial 2, but includes at least a portion 3 made of a magneticmaterial, which has a suitable phase transition temperature, e.g. in theinterval 0-100° C. Alternatively, an essential portion of the magneticcircuit or the entire circuit is of the magnetic material with thesuitable phase transition temperature.

The temperature-varying device 5 is provided for varying the temperaturein the portion made of the magnetic material with the suitable phasetransition temperature alternately above and below a magnetic phasetransition temperature of the magnetic material preferably with afrequency of about or above 1 Hz. Temperature-varying devices that canbe used in the present information will be described later in thisdescription. Examples of magnetic phase transition temperatures are theCurie temperature and the Néel temperature.

The rapid variation of temperature above and below the phase transitiontemperature causes drastic changes of the permeability of the magneticmaterial and thus a rapid variation of the magnetic resistance orreluctance of the magnetic circuit 1. More concretely, the magnetizationis varied rapidly when a constant magnetic field is applied.

Provided that a magnetic flux is provided in the magnetic circuit 1, therapid variation of the reluctance will modulate the magnetic flux,thereby obtaining a rapidly varying magnetic flux in the magneticcircuit 1. As a result a magnetomotive force and an alternating currentare obtained in the coil 7.

The magnetic flux can be provided by a permanent magnet or, as in FIG.1, by an electromagnet.

The current for the electromagnet is advantageously taken from thecurrent induced in the coil in a novel manner. To this end, a capacitor9 is connected in parallel with the coil 7 to thereby form a resonantelectric circuit 11, wherein the frequency of the temperature variationabove and below the phase transition temperature of the magneticmaterial is adjusted to optimize the resonant energy transfer to theresonant electric circuit 11.

Advantageously, the ratio of the resonance frequency of the resonantelectric circuit 11 and the frequency of the temperature variation aboveand below the phase transition temperature of the magnetic material isapproximately ½ or n/2, where n is a positive integer.

Hereby, a single coil will be used for the transformation of heat toelectric energy and for providing a magnetic flux in the magneticcircuit 1. Such fields of alternating directions provides for a morecost efficient apparatus.

A part, e.g. a major part, of the current/charge induced in one half ofa first thermal cycle is stored by the capacitor 9 and is used in thefollowing half of the first thermal cycle to generate a magnetic flux inthe magnetic circuit 1. This first thermal cycle corresponds to one halfof an electric cycle. The procedure is repeated through a second thermalcycle with current and voltage 180 degrees phase shifted.

In order to be capable of controlling the resonance frequency, and thereactance of the electric circuit formed by the coil 7 and the capacitor9, a fully controllable load or power electronic circuit device 13 isconnected over the capacitor 9. Preferably, the load has an inductivecomponent/capacitive component and a resistive component, each of whichbeing separately and individually controllable. Advantageously, the loadcan be used to adjust the active power. A suitable control device 15 isprovided for controlling the load 13. Different measurement devices,such as a thermo sensor 16, current transformers 17, and a voltagetransformer 18 may be provided to supply the control device 15 withsuitable measurement data. The thermo sensor 16 may supply the controldevice 15 with temperature data instantaneously measured in or at themagnetic material with the suitable phase transition temperature or inor at the temperature-varying device 5. The transformers 17, 18 maysupply the control device 15 with voltage and current datainstantaneously measured in the resonant electric circuit 11.

Hereby, the amplitude and phase of the impedance of the load can bedynamically controlled. The frequency and period of a variation of theimpedance is controllable, and so is the frequency and period of theresonance of the resonant circuit 11.

Further, the control device 15 may be configured to control theamplitude and frequency of the rapid variation of the temperature aboveand below the phase transition temperature.

Still further, the control device 15 may be provided to initiate theoperation of the generator device, i.e. to start the resonantoscillations, e.g. by delivering a current pulse to the magnetic circuit1.

The generator device described above may be compact without essentiallyany moving parts and without any conversion to or from mechanicalenergy. Operation is easily controlled and efficiency may be optimizedby adjusting the output power during a cycle.

It shall be appreciated that a plurality of the generator device of FIG.1 can be connected together to form a multiphase generator device. Forinstance three generator devices of FIG. 1 may be connected together andbe phase-shifted 120° with respect to one another to form a three-phasegenerator device.

With reference next to FIG. 2, a multiphase thermomagnetic generatordevice according to a further embodiment of the present invention willbe shortly overviewed.

This embodiment uses the same reference numerals as the previousembodiment to indicate similar parts and details.

Three only schematically indicated magnetic circuits 1 are provided,each of which being of the kind described with reference to FIG. 1 andeach of which being operatively connected to a respective LC circuit 11including a winding or coil 7 and a capacitor 9 connected in parallel.The resonance frequency each of the LC circuits 11 is as beforeessentially similar to the frequency of the temperature variation ascreated by the temperature-varying device 5.

The temperature-varying device 5 comprises an outer part, which includesa first external pipe arrangement 21, in which hot fluid is circulatedby a feed pump 22, and a second external pipe arrangement 23, in whichcold fluid is circulated by a feed pump 24. The hot and cold fluids ofthe outer part are entirely isolated from each other as well as from thematerial of the magnetic circuits 1.

The hot fluid in the first external pipe arrangement 21 transfers heatto fluid in a first intermediate pipe arrangement 25 via a first heatexchanger 26 and the cold fluid in the second external pipe arrangement23 transfers cold to fluid in a second intermediate pipe arrangement 27via a second heat exchanger 28. Each of the first and secondintermediate pipe arrangements 25, 27 is connected between a first valvearrangement 29 and a second valve arrangement 30 to transport fluid fromthe first valve arrangement 29 to the second valve arrangement 30. Thefirst and second valve arrangements 29, 30 are advantageously based onrotating valves, examples of which being disclosed in FIGS. 3-4.

It shall be appreciated that the outer part may be exchanged for anyother kind of arrangement for transferring heat and cold in the heatexchangers 26 and 28. For instance, heat may be transferred to fluid inthe first intermediate pipe arrangement 25 in the first heat exchanger26 via an incinerator, hot sand, a solar heating panel, or similar.

Finally, a first 31, a second 32, and a third internal pipe arrangementare each connected between the second valve arrangement 30 and the firstvalve arrangement 29 via a respective one of the magnetic circuits 1.

A single fluid is flowing in the inner part of the temperature-varyingdevice 5, which comprises the intermediate and internal pipearrangements and the first and second valve arrangements. The inner partthus provides a closed fluid loop.

The second valve arrangement 30 is provided for alternately switchinghot fluid from the first intermediate pipe arrangement 25 and cold fluidfrom the second intermediate pipe arrangement 27 into each one of thefirst, second and third internal pipe arrangements 31, 32, 33,preferably with a 120° phase shift there in between. Thus, the secondvalve arrangement 30 “chops” the hot and cold fluids and forms trains ofalternating hot and cold fluid pulses, which are fed into each of theinternal pipe arrangements. As the hot and cold fluid pulses pass by, orthrough holes in, a magnetic material of the magnetic circuits 1, themagnetic material will be alternately heated above and cooled below thephase transition temperature as was described above in connection withthe embodiment of FIG. 1.

The terms “hot fluid” and “cold fluid” as used in the present text areintended to indicate “fluid having a temperature above the phasetransition temperature of the magnetic material of the portion 3 of themagnetic circuit” and “fluid having a temperature below the phasetransition temperature of the magnetic material of the portion 3 of themagnetic circuit”, respectively.

The magnetic material can be provided as parallel sheets or platespreferably arranged in parallel to one another, granulates, smallspheres, wires, fabrics or similar allowing the fluid, in a laminar orturbulent flow, to exchange heat with the magnetic material with largecontact surface.

After having passed the magnetic material the temperature variationbetween the hot and cold fluid pulses is smaller and smoother. Thetrains of hot and cold fluid pulses are then returned in the respectiveinternal pipe arrangements 31, 32, 33 to the first valve arrangement 29,which is synchronized with the trains of hot and cold fluid pulses.

The first valve arrangement 29 is thus provided for alternatelyswitching the hotter fluid pulses from the first, second and thirdinternal pipe arrangements 31, 32, 33 into the first intermediate pipearrangement 25 and the colder fluid pulses from the first, second andthird internal pipe arrangements 31, 32, 33 into the second intermediatepipe arrangement 27. Hereby, the hotter and colder fluid pulses arereturned to the respective intermediate pipe arrangement, from whichthey were originating. The fluid in the first intermediate pipearrangement 25 is then returned to the first heat exchanger 26 in orderto be heated again and the fluid in the second intermediate pipearrangement 27 is then returned to the second heat exchanger 28 in orderto be cooled again.

The fluid in the inner part is driven in a single direction by feedpumps 34, 35.

The rotating valves 29, 30 of FIG. 2 are advantageously mounted on asingle shaft to be rotated simultaneously/synchronously with a suitablephase shift there in between.

In an alternative embodiment the first valve arrangement 29,particularly where the temperature difference between the hotter andcolder fluid pulses is low, the hotter and colder fluid pulses from thefirst, second and third internal pipe arrangements may not have to beswitched back into the second and first intermediate pipe arrangements.Thus, the first valve arrangement 29 may be dispensed with, and anotherkind of passive distribution or mixing arrangement may be used insteadin order to return the fluids to the second and first intermediate pipearrangement. If an open circuit is used the fluids do not have to bereturned.

An example of a rotating valve 40 to be used in the present invention isdisclosed in FIG. 3. A hollow cylinder or cylindrical casing 41 houses asymmetrically arranged rotatable shaft 42, to which two blades or wings43 are fixedly attached. The two blades 43, which preferably arethermally isolating, are provided in close fit with the cylindricalcasing 41 and define two essentially separated and identicalcompartments or chambers 44 a-b of the rotating valve. The shaft 42 andthe blades 43 are advantageously mounted in the cylindrical casing 41 bymeans of bearings and means, e.g. an electric motor, is provided toapply a driving torque on the shaft 42. Alternatively, the drivingtorque is applied via an internal propeller or similar attached to theshaft 42 and/or via self-propulsion in the flow.

A first one 44 a of the chambers is connected to the first intermediatepipe arrangement 25 and is configured to receive hot fluid from there,and the second one 44 b of the chambers is connected to the secondintermediate pipe arrangement 27 and is configured to receive cold fluidfrom there. To this end a respective half-circular cover plate (notexplicitly illustrated) is fixedly provided with respect to the shaft 42and the blades 43 at a respective axial end of the rotating valve,wherein the cover plates rotate with the shaft 42 and the blades 43during operation. A first one of the cover plates covers the secondchamber 44 b and prevents hot fluid from entering the second chamber 44b at the axial end connected to the first intermediate pipe arrangement25 and a second one of the cover plates covers the first chamber 44 aand prevents cold fluid from entering the first chamber 44 a at theaxial end connected to the second intermediate pipe arrangement 27.

In the illustrated example six outlets 45 a-f are arrangedcircumferentially in the casing 41, preferably with equal distancesbetween one another. During operation, the shaft 42 and the blades 43are steadily rotated with respect to the casing 41 and the outlets 45a-f with steady incoming axial flows of hot and cold fluid,respectively, into chambers 44 a-b, which are also rotated with respectto the casing 41 and the outlets 45 a-f, thereby causing each of theoutlets 45 a-f to alternately output hot and cold fluid pulses. Therotational speed controls the wavelength and frequency of the hot andcold fluid pulses and the angular separation of the outlets controls thephase.

In FIG. 3, the blades 43 are oriented so that hot fluid is outputthrough the outlets 44 a-c from the chamber 44 a, which receives hotfluid, whereas cold fluid is output through the outlets 44 d-f from thechamber 44 b, which receives cold fluid. As the blades are rotatedclockwise, the outlet 45 a switches from outputting hot fluid tooutputting cold fluid, while, the outlet 45 d switches from outputtingcold fluid to outputting hot fluid, etc.

The rotating valve has generally at least two outlets, and haspreferably an even number of outlets in order to minimize pressuregradients and variations. For the particular embodiment shown in FIG. 2,the rotating valve for alternately outputting hot and cold fluid hasthree outlets.

Further, the rotating valve may comprise four or more blades, and, as aconsequence, four or more chambers alternately connected to hot and coldfluid. Hereby, the frequency of the temperature variation can beincreased.

While the above rotating valve has been described as receiving hot andcold fluid in different chambers and outputting hot and cold fluidalternately in each of a plurality of outlets at the circumference ofthe rotating valve, it is equally applicable to a rotating valve such asthe rotating valve 29 of FIG. 2, i.e. a rotating valve which receiveshot and cold fluid alternately in each of a plurality of inlets at thecircumference of the rotating valve and which outputs hot and cold fluidaxially via different chambers.

To fit the embodiment of FIG. 2 each of the rotating valves shouldadvantageously have three inlets/outlets at the circumference thereof.

An alternative example of a rotating valve 40′ to be used in the presentinvention, which is disclosed in FIG. 4, differs from the rotating valveof FIG. 3 in that the blades 43 are exchanged for an elliptic disc 43′,which is fixedly attached to the shaft 42 in an inclined position. Theelliptic disc 43′ is arranged in close fit with the cylindrical casing41, to define the first and second chambers 44 a-b. The elliptic disc43′ is arranged at an axial position and with an inclination angle suchthat each of the outlets/inlets 45 a-f at the circumference of thecylindrical casing 41 is alternately in fluid connection with the firstand second chamber 44 a-b as the shaft 42 and the elliptic disc 43′ arerotated with respect to the cylindrical casing 41. Each of the axialends of the rotating valve 40′ may, similar as those of the rotatingvalve 40 of FIG. 3, be connected to a respective one of the first andsecond intermediate pipe arrangement 25, 27.

The elliptic plate might be fabricated by cutting it from a predrilledsolid cylinder having a diameter slightly less than the inner diameterof the cylindrical casing.

A large number of outlets minimizes possible pressure variationsassociated with the elliptic disc 43′ sweeping by an outlet. During thepeak of such an event the disc 43′ may, depending on the actual designchosen, cover either the full outlet area (some unsteadiness has betolerated in the outlet flow) or only part of it (some mixing has to betolerated in the outlet flow). Due to symmetry, the forces acting on thedisc 43′ and the shaft 42 are foreseen to be small, provided the fluidpressure is equal in the two chambers. Also, for the same reason, asmall separation between the disc 43′ and the wall of the cylindricalcasing 41 may be allowed, reducing or eliminating solid-to-solid contactforces with only negligible amounts of fluid being mixed.

The disc 43′ may be suitable reshaped, e.g. by means of bulging,bending, and/or twisting, thereby requiring a shape other thanelliptical, for self-propulsion of the shaft/disc combination in theflow.

The rotating valves as have been described with reference to FIGS. 3 and4 are capable of distributing industrial scale amounts of fluids withdifferent characteristics to a common outlet (or several common outlets)with minimal mixing on a sub-second scale. The rotating valves allow fora steady fluid flow with minimal disturbance from switching, minimalswitching power demand, and a long lifetime with the ability to switchmillions of cycles. Conventional valves and piston pumps either are tooslow, too disruptive (flow stop, pressure waves), power demanding and/orwear out after rather short a number of cycles.

The rotating valves can thus not only be used for the application ofproducing trains of hot and cold fluid pulses, but are applicable for alarge variety of industrial processes which involve alternatingdistribution of fluid with different characteristics into a commonoutlet, keeping the fluids separated with minimal mixing at a rate of afew cycles per second, continuously for several years. The fluids havepreferably roughly similar fluid properties concerning e.g. density,viscosity, etc. They may consist of different substances, like water andethanol, or of the same substance in different property states, like hotand cold water.

By the temperature-varying device 5 as being described above withreference to FIG. 2, thermal cycling in a quasi-continuous or continuousmanner is enabled. By means of having the fluid to circulate in auni-directional closed loop the traditional disruptive and energeticallyinefficient cycling using valves switching on and off the fluid flow isentirely avoided. Nevertheless, the rotating valves may be exchanged foran arrangement of ordinary valves that are opened or closed in order toobtain the operation depicted above.

The smooth continuous thermal cycling described above may equally wellbe implemented in a two-phase generator device or even in a single-phasegenerator device. In the latter case still two rotating valves arerequired, each having two inlets/outlets arranged circumferentially atthe valve casing. One of the alternately hot and cold fluid flows outputfrom the outlets are either connected directly to the return rotatingvalve phase or is appropriately delayed and then brought together withthe other one of the alternately hot and cold fluid flows to obtain asingle flow of alternately hot and cold fluids with a double flow rate.Pressure differences between the different ones of the alternately hotand cold fluid flows may advantageously be balanced by an additionalcirculation pump.

The fluid used in the inner closed loop may be water or any other fluid,optionally to which additives have been added. Additives that reducecorrosion with respect to the magnetic material may be added. Otheradditives, such as thermal salts or similar with a phase transitionadjusted to the phase transition temperature of the magnetic material tothereby increase the heat capacity of the fluid, may additionally oralternatively be added.

Preferably, the magnetic material is gadolinium or an alloy comprisinggadolinium. Alternatively, the magnetic material with the suitable phasetransition temperature is any of bismuth, zinc, antimony, tellurium,selenium, lead, silicon, germanium, tin, magnesium, manganese, arsenic,nickel, lanthanum, gallium, phosphorus, calcium, barium, strontium,ytterbium, iron or any alloy or compound thereof. Note that not all ofthe above elements are magnetic, but can in such instance be used with amagnetic material to modify its magnetic phase transitioncharacteristics. Some of the above elements may be present in themagnetic material of the invention as an oxide, or in any other kind ofchemical compound.

The multiphase generator device of FIG. 2 comprises furtheradvantageously a power conversion device connected to the capacitors 9of the three generator units or phases at the output. The coils 7 andthe power conversion device are controlled to match the cycle of thethermal variation and to thereby enable optimum energy to be tapped fromthe circuit. The power conversion device may comprise an AC/DC or AC/ACfrequency converter or a power electronic converter including a currentor voltage source converter 36, which encompasses a rectifier and aninverter at the DC side of the rectifier.

Additionally, or alternatively, a battery is connected at the DC side.This is particularly advantageous if the power output from the generatoris very fluctuating.

A transformer 37 is connected to the output of the voltage sourceconverter 36 to transform the output voltage and frequency of about 1 kVand 1 Hz from the multiphase generator to a frequency and a voltage (50Hz, 10 kV) suitable for normal grid connection. The rating of theequipment is typically larger than 1 kW.

Preferably, the generator device of the present invention is providedfor electric power generation in the range of 100 kW to 50 MW, and morepreferably in the range of 1-5 MW. Several generator devices may bearranged together as modules to generate electric powers of about 10-50MW.

A plant control system may be provided for measuring the performance ofthe generator device and optimizing the power output, e.g. as wasdescribed with reference to FIG. 1. Further sensor devices (notillustrated) may be provided for measuring flow rates of the temperaturevarying fluid, and the control system may be provided for controllingthe flow rates of the temperature varying fluids as well as the speedsof the rotating valves.

The generator device described above is capable of performing activecontrol of the oscillations in the electric resonance circuits in orderto obtain optimum correlation with the temperature changes of themagnetic material. Active power can be output in optimum synchronizationwith the cycles.

The provision of inner and outer parts separated from one another is notnecessary. For instance, the hot fluid may be taken directly from e.g. a(hydro)thermal bore hole and be returned via a secondary or return borehole.

It shall be appreciated that the temperature varying device describedabove may be used in virtually any kind of thermomagnetic generatordevice including those having a permanent magnet for magnetizing themagnetic circuit as well as those having a separate coil and currentfeeding, thereby forming an electromagnet for magnetizing the magneticcircuit.

The generator device of the present invention is advantageously used forthe generation of electric power by using any of waste heat, combustionheat, thermal storage reservoir energy, geothermal energy, solarradiation, solar thermal energy, ocean thermal energy, or energy fromnuclear reactions.

The invention claimed is:
 1. A multiphase generator device for converting thermal energy to electric energy comprising a plurality of generator units, each of the generator units comprising: a magnetic circuit including at least a portion made of a magnetic material; a coil arranged around said magnetic circuit, in which electric energy is induced in response to a varying magnetic flux in said magnetic circuit; and a magnetic flux generator configured to create a magnetic flux in said magnetic circuit, wherein a temperature-varying device configured to vary a temperature in said portion made of the magnetic material alternately above and below a phase transition temperature of the magnetic material to thereby vary the reluctance of the magnetic circuit; and said temperature-varying is configured to alternately pass hot and cold fluid by, or through holes in, the portion made of the magnetic material of the magnetic circuits in a single direction to thereby vary the temperature in said portion made of the magnetic material alternately above and below the phase transition temperature of the magnetic material, wherein said temperature-varying device comprises a rotating valve having an input provided for receiving hot fluid, an input provided for receiving cold fluid and a plurality of outputs, each of which being provided for alternately outputting hot and cold fluid to a respective one of the plurality of generator units.
 2. The multiphase generator device according to claim 1, wherein said temperature-varying device is configured to circulate said hot and cold fluid in a closed fluid loop.
 3. The multiphase generator device according to claim 2, wherein said temperature-varying device is configured to circulate said hot and cold fluid quasi-continuously.
 4. The multiphase generator device according to claim 1, wherein said rotating valve comprises a cylindrical casing; a shaft arranged symmetrically in said casing; a blade or disc arrangement fixedly attached to said shaft and defining essentially separated chambers within the casing; and a plurality of output ports arranged along the circumference of said casing, wherein the chambers are alternately in fluid connection with each of the output ports in response to rotation of said shaft and said blade or disc arrangement with respect to said casing; one of the chambers is connected to a first intermediate pipe arrangement and is configured to receive hot fluid from there, and the other one of the chambers is connected to a second intermediate pipe arrangement and is configured to receive cold fluid from there, thereby feeding alternately hot and cold fluid through each one of the plurality of output ports.
 5. The multiphase generator device according to claim 4, wherein said temperature-varying device comprises a second valve arrangement, which is provided, for each of the magnetic circuits, to receive the alternately hot and cold fluid after having been exposed to the respective portion made of the magnetic material of the magnetic circuit and to switch the hot fluid therein into said common first pipe and the cold fluid therein into said common second pipe.
 6. The multiphase generator device according to claim 5, wherein said second valve arrangement includes a rotating valve.
 7. The multiphase generator device according to claim 6, wherein the rotating valve of said second valve arrangement comprises a cylindrical casing; a shaft arranged symmetrically in said casing; a blade or disc arrangement fixedly attached to said shaft and defining essentially separated chambers within the casing; and a plurality of input ports arranged along the circumference of said casing, wherein the chambers are alternately in fluid connection with each of the input ports in response to rotation of said shaft and said blade or disc arrangement with respect to said casing; each of the input ports is connected to receive the alternately hot and cold fluid; one of the chambers is arranged to receive the hot fluid from the input ports and is connected to output the hot fluid into said common first pipe; and the other one of the chambers is arranged to receive the cold fluid from the input ports and is connected to output the cold fluid into said common second pipe.
 8. The multiphase generator device according to claim 4, wherein said temperature-varying device comprises a first external pipe, in which hot fluid is circulated, said first external pipe and said common first pipe being arranged in a first heat exchange section in order to transfer heat from the hot fluid in the first external pipe to the hot fluid in the common first pipe.
 9. The multiphase generator device according to claim 4, wherein said temperature-varying device comprises a second external pipe, in which cold fluid is circulated, said second external pipe and said common second pipe being arranged in a second heat exchange section in order to transfer cold from the cold fluid in the second external pipe to the cold fluid in the common second pipe.
 10. The multiphase generator device according to claim 1, wherein said plurality of generator units is three, and wherein said generator units are provided for delivering AC currents which are 120° phase shifted with respect to one another.
 11. The multiphase generator device according to claim 1, wherein each of said generator units comprises a capacitor connected in parallel with the coil of the generator unit to thereby repeatedly create a magnetic flux in the magnetic circuit of the generator unit.
 12. The multiphase generator device according to claim 1, wherein each of said generator units comprises a winding arranged around the magnetic circuit and a current source connected to the winding and configured to flow a current through the winding to thereby create a magnetic flux in the magnetic circuit of the generator unit.
 13. The multiphase generator device according to claim 1, wherein the magnetic circuit of each of said generator units comprises a permanent magnet for creating a magnetic flux in the magnetic circuit.
 14. The multiphase generator device according to claim 1, wherein each of said generator units comprises a controllable load connected to the coil of the generator unit; and a control device for controlling the load.
 15. The multiphase generator device according to claim 1, wherein the thermal energy comprises any of waste heat, combustion heat, thermal storage reservoir' energy, geothermal energy, solar radiation, solar thermal energy, ocean thermal energy, or energy from nuclear reactions.
 16. A method for operating a multiphase generator device for converting thermal energy to electric energy comprising a plurality of generator units, said method comprising: converting thermal energy to electric energy in each of the generator units, each of which comprising a magnetic circuit including at least a portion made of a magnetic material and a coil arranged around the magnetic circuit, the method comprising the step of: varying the temperature in the portion made of the magnetic material of each of the generator units alternately above and below a phase transition temperature of the magnetic material to thereby vary the reluctance of the magnetic circuit; creating a magnetic flux in the magnetic circuit of each of the generator units; and inducing electric energy in the coil of each of the generator units in response to a varying magnetic flux in the magnetic circuit, around which the coil is arranged, wherein the varying of the temperature in the portion made of the magnetic material of each of the generator units alternately above and below a phase transition temperature is obtained by alternately passing hot and cold fluid by, or through holes in, the portion made of the magnetic material of the magnetic circuit in a single direction, characterized in that the alternately passing of hot and cold fluid by, or through holes in, the portion made of the magnetic material of the magnetic circuit of the generator units is obtained by receiving hot fluid at an input of a rotating valve, receiving cold fluid at another input of the rotating valve, and alternately outputting at a plurality of outputs hot and cold fluid to a respective one of the plurality of generator units.
 17. The method according to claim 16, wherein said hot and cold fluids are circulated in a closed fluid loop.
 18. The method according to claim 16, wherein said hot and cold fluids are circulated in a quasi-continuous manner.
 19. The method according to claim 16, wherein the thermal energy comprises any of waste heat, combustion heat, thermal storage reservoir' energy, geothermal energy, solar radiation, solar thermal energy, ocean thermal energy, or energy from nuclear reactions. 